Microscopy Tool Adds ‘Color’ with Nanoscale Resolution

Nanoscale objects can now be examined in full color, thanks to a new microscopy tip that delivers chemical details with a resolution once thought impossible. The nanotool could help scientists probe solar-to-electric energy conversion at its most fundamental level.

Scientists can make and manipulate nanoscale objects with increasing control, but until now have been limited to black-and-white imagery for examining those objects. Information about nanoscale chemistry and interactions with light — the atomic-microscopy equivalent to color — has been out of reach.

Lawrence Berkeley National Laboratory (Berkeley Lab) researchers discovered that replacing the usual atomic force microscope tip with what they named a campanile tip — meaning “bell tower” and inspired by the clock tower on the UC Berkeley campus — provides a wealth of optical data in addition to the spatial mapping. They compared it to going from a blurry 1950s black-and-white television picture to today’s high-definition color displays.

A new microscopy tool promises to revolutionize nanoscale imaging. Left, a design schematic of what the UC Berkeley team named the “campanile” microscopy tip. Right, an electron micrograph of the tip and, inset, the campus campanile bell tower for which it is named. Images courtesy of Lawrence Berkeley National Lab.

"We've found a way to combine the advantages of scan/probe microscopy with the advantages of optical spectroscopy," said Alex Weber-Bargioni, a scientist at the Molecular Foundry, a nanoscience center at Berkeley Lab. "Now we have a means to actually look at chemical and optical processes on the nanoscale where they are happening."

"If you want to characterize materials, particularly nanomaterials, the way it's traditionally been done is with electron microscopies and scan/probe microscopies because those give you really high, subatomic spatial resolution," said James Schuck, a nano-optics researcher at the Molecular Foundry. "Unfortunately, what they don't give you is chemical, molecular-level information."

For chemical information, researchers typically turn to optical or vibrational spectroscopy. The way a material interacts with light is dictated in large part by its chemical composition, but for nanoscience the problem with doing optical spectroscopy at relevant scales is the diffraction limit.

Electromagnetic fields are enhanced in the gap as the campanile uses surface plasmons to squeeze light beyond the diffraction limit, as shown in these simulations.

To get around this constraint, scientists employ near-field light, which decays exponentially away from an object, making it hard to measure, but contains very high resolution — much higher than normal, far-field light.

"The real challenge to near-field optics, and one of the big achievements in this paper, is to create a device that acts as a transducer of far-field light to near-field light,” Schuck said. “We can squeeze it down and get very enhanced local fields that can interact with matter. We can then collect any photons that are scattered or emitted due to this interaction, collect in the near field with all this spatial frequency information, and turn it back into propagating, far-field light."

The trick for that conversion is to use surface plasmons. Plasmons on two surfaces separated by a small gap can collect and amplify the optical field in the gap, making a stronger signal for scientists to measure.

Researchers have exploited these effects to make near-field probes with a variety of geometries, but the experiments typically require painstaking optical alignment, suffer from background noise, only work for narrow frequency ranges of light, and are limited to very thin samples.

Using the campanile tip, Berkeley Lab researchers take “color” images with nanoscale resolution. A photovoltaic indium-phosphide nanowire is easy to see in a black-and-white electron micrograph (left), but chemical information has low resolution in a normal confocal micrograph (right). The campanile tip reveals both the shape and chemistry of a nanowire (center.)

They transcended these limitations using a tapered, four-sided tip fabricated onto the end of an optical fiber. Two of the campanile’s sides are coated with gold, and the two gold layers are separated by just a few nanometers at the tip. The three-dimensional taper enables the device to channel light of all wavelengths down into an enhanced field at the tip. The size of the gap determines the resolution.

"That's the beauty of these tips," Schuck said. "You can just put them on the end of an optical fiber, and then it's just like using a regular AFM. You don't have to be a super near-field jock anymore to get this type of data."

They developed the tool to study indium-phosphide nanowires for conversion of solar energy to electricity. They found that the nanowires were not the homogeneous objects previously thought, but instead had varying optoelectronic properties, which could radically alter how sunlight is converted to electricity. They also found that photoluminescence was seven times stronger in some parts of a nanowire than others. This is the first time anyone has measured these events on such a small scale.

"Details like this about indium-phosphide nanowires are important because if you want to use these suckers for photocatalysis or a photovoltaic material, then the length scale at which we're measuring is where everything happens,” Weber-Bargioni said.

They are using it for imaging and spectroscopy but anticipate it for many other applications, Schuck said.

The findings were reported in Science (doi: 10.1126/science.1227977).

For more information, visit: www.lbl.gov